Select a flux. Much like the solder, the flux needs to be specialised for aluminum or for The mixture of metals you plan to join. The best solution is to purchase the flux through the same source as your solder, as They can be very likely intended to operate jointly. https://whatsaspoileronacar15814.sharebyblog.com/27180297/the-2-minute-rule-for-types-of-solder-to-use-on-aluminum-radiator